1. Field of the Invention
The present invention relates to a method for preparing a solid device for use as an oxide superconducting material, and more particularly to a method for preparing a solid device, the surface of which is utilized for oxide superconducting material wherein an important improvement is imparted to the properties of the material at the surface or portion close to the surface, to provide a highly reliable surface-utilizing-device.
2. Description of the Related Art
Recently, considerable attention has been directed toward oxide superconducting materials. This began with the development of a Ba-La-Cu-O type of oxide superconducting material in the IBM research laboratories in Zurich, Switzerland. In addition to this, an yttrium type of oxide superconducting material is also known, which has provided the obvious possibility for the practical application of a solid device at the temperature of liquid nitrogen.
On the other hand, superconducting materials using metals such as Nb.sub.3 Ge have been well known conventionally. Trials have been conducted in fabricating solid devices such as the Josephson element using this metal superconducting material.
After a dozen years of research, a Josephson device using this metal is close to being realized in practice. However, the temperature of this superconducting material at which the electrical resistance becomes zero (which is hereinafter referred to as Tco) is extremely low, that is 23.degree. K., so that liquid helium must be used for cooling. This means that practical utility of such a device is doubtful.
With a superconducting material made of this metal, the components on both the surface and in the bulk of the material can be made completely uniform because all the material is metal.
On the other hand, when the characteristics of the oxide superconducting material which has been attracting so much attention recently are examined, a deterioration of the characteristics (lowering of reliability) is observed at the surface or portion close to the surface (roughly 200 .ANG. deep), in comparison with the bulk of the material.
It has been possible to prove experimentally that the reason for this is that the oxygen in the oxide superconducting material can be easily driven off.
Further, when observed with an electron microscope, an empty columnar structure is seen with an inner diameter of 10 .ANG. to 500 .ANG., and usually 20 .ANG. to 50 .ANG. in the oxide superconducting material, and in other words, the oxide superconducting material is found to be a multiporous material having indented portions in micro structure. For this reason the total area at the surface or portion close to the surface is extremely large, and when this oxide superconducting material is placed in a vacuum, the oxygen is broken loose as if absorbed gas was driven off.
The basic problem is determined that whether the material has superconducting characteristics or simply normal conducting characteristics is dependent on whether the oxygen is present in ideal quantities or is deficient.